Fixed #21552 -- Allowed the use of None for the iexact lookup.

Thanks Anubhav Joshi for the documentation.
Fixed #21632 -- Docs: Removed example with callable as query parameter

Using callables as query parameters is undocumented and not working,
so this changes an example from the ForeignKey.limit_choices_to
documentation that uses it.
Fixed #11629 -- Deprecated callable arguments to queryset methods.

Callable arguments were an untested and undocumented feature.
Fixed #21386 -- Removed admindocs dependence on sites framework

* Removed ADMIN_FOR setting and warn warning
* Group view functions by namespace instead of site
* Added a test verifying namespaces are listed

Thanks to Claude Paroz for reviewing and ideas for improvement.
Simplied some code in django.conf.__init__.
Fixed #21613 -- Removed a hardcoded input id from the tutorial.

Thanks Bruno Brouard for the report.
Fixed #21585 -- Updated some links to 3rd party database projects.

Thanks graeme.perrow at sap.com for the original report.